1. Field of the Invention
The present invention relates to an image forming apparatus, in particular, a controlling method used in an image forming apparatus that electrophotographically forms images and fixes the formed image onto a recording material.
2. Description of the Related Art
In a conventional image forming apparatus, a toner image on an image bearing member is transferred onto a recording material by transferring means such as a transferring roller and the recording material is led via a conveyance guide to a nip portion of a fixing portion, which fixes the toner image. However, there may be a state in which a rear end portion of the recording material has not passed through the transferring portion when a leading end portion of the recording material is led to the nip portion of the fixing portion.
In this case, the recording material is conveyed by both the fixing portion and the transferring portion. Therefore, if a recording material conveying speed of the fixing portion is faster than a recording material conveying speed of the transferring portion, the recording material becomes stretched resulting in deterioration of image quality such as a color drift or a transferring shake in a transferring step. In contrast, if the recording material conveying speed of the transferring portion is faster than the recording material conveying speed of the fixing portion, a loop (or curve) becomes too large resulting in lack of space for maintaining an appropriate loop shape. Therefore, there may be a problem that a surface of an unfixed image is rubbed in the apparatus.
Therefore, as described in Japanese Patent Application Laid-Open No. H07-234604, a loop is usually formed in the recording material before an inlet of the fixing portion, and an amount of the loop is adjusted (hereinafter, referred to as “loop control”) so as to suppress the deterioration of image quality in the transferring step or a contact of the unfixed image with a structural element of the apparatus between the transferring and fixing stages. Specifically, a sensor is disposed for sensing an upper limit and a lower limit of the amount of the loop formed in the recording material. When the upper limit of the loop amount is sensed, a speed of a drive source (motor) of a drive system for the fixing means is increased by a constant amount. In addition, when the lower limit of the loop amount is sensed, a speed of the drive source of the drive system for the fixing means is decreased by a constant amount.
On the other hand, Japanese Patent Application Laid-Open No. H10-340012 proposes an image forming apparatus including a loop detecting sensor for detecting a loop in a recording material, which is disposed at a conveyance guide portion between the fixing portion and transferring portion. Based on a result of the detection, the recording material conveying speed of the fixing portion or transferring portion is switched so that the loop amount of the recording material becomes a value within a predetermined range.
In addition, Japanese Patent Application Laid-Open No. 2001-282072 proposes an image forming apparatus including two detecting sensors for detecting a loop amount of a recording material conveyed from the transferring portion to a fixing nip portion of the fixing device. If one of the sensors detects a loop amount, the recording material conveying speed is controlled in the direction for decreasing the loop amount. If the other sensor detects a loop amount, the recording material conveying speed is controlled in the direction for increasing the loop amount. Thus, the loop amount of the recording material can be controlled to be a value within a constant range.
However, if the loop control described in each of Japanese Patent Application Laid-Open No. H07-234604, Japanese Patent Application Laid-Open No. H10-340012, and Japanese Patent Application Laid-Open No. 2001-282072 is performed, the following problem may occur.
The loop control described in Japanese Patent Application Laid-Open No. H07-234604 adjusts the loop amount by switching the drive system drive source (motor) of the drive system for the fixing portion between two speeds, one of which is a predetermined high speed (H) and another one of which is a predetermined low speed (L). If the fixing portion is driven at the high speed (H), the loop amount of the recording material is decreased. In contrast, if the fixing portion is driven at the low speed (L), the loop amount of the recording material is increased.
There may be a case where the recording material conveying speed of the fixing portion is different despite the same r.p.m. of the motor. For instance, a recording material conveying roller of the fixing portion may be deteriorated gradually along with an increase of a accumulative used amount of the image forming apparatus, or a surface characteristic of the recording material may be changed due to a variation of environment in which the image forming apparatus is installed. This variation of the recording material conveying speed may occur in the transferring portion, too. Such a variation factor of the recording material conveying speed should be taken into account, and the high speed (H) should be preset to be such a value that the loop amount of the recording material can be sufficiently small. In addition, the low speed (L) should be preset to be such a value that the loop amount of the recording material can be sufficiently large.
Therefore, a speed difference between the high speed (H) and the low speed (L), i.e., a speed control range should be secured widely considering influences of endurance variations of the fixing means and the transferring means, using environment of the image forming apparatus, a type of the recording material and the like.
A fixing unit provided to the image forming apparatus is usually designed to have a life period shorter than that of a main body of the image forming apparatus, and hence the fixing unit is replaced at the end of its life with a new fixing unit. If the fixing unit has a short period of life, it is considered that a difference between the recording material conveying speed when the fixing unit is new and the recording material conveying speed when the fixing unit is close to the end of its life is small. Therefore, a speed difference between the high speed (H) and the low speed (L) of the motor can also be decreased.
However, if the fixing unit has a long period of life, it is considered that a difference between the recording material conveying speed when the fixing unit is new and the recording material conveying speed when the fixing unit is close to the end of its life is large. Therefore, it is necessary to set the speed difference between the high speed (H) and the low speed (L) of the motor to be a large value.
In this case, the loop control may cause hunting. As a result, the loop amount of the recording material is hardly controlled within a desired range, and hence gross unevenness corresponding to the switching of the fixing speed or unevenness of overhead transparency (OHT) may occur. In a worse case, paper wrinkle due to unstable conveying, stretching between the transferring means and the fixing portion, image abrasion due to an increase of the loop, and color drift of each color due to a variation of a load on the recording material may also occur.